Telephone voice communication systems are ubiquitously deployed over most of the United States and a good portion of the world. Telephone voice communication uses the frequency band 300 Hz to 3300 Hz. A copper pair connects the end user's premises to the serving central office. This provides satisfactory voice signal transmission. As the need for data communication increases, transmission of data signals from the end user's premises to the serving central office, becomes a problem. Using voice band modems, the user can establish data communication up to about 4.8 kbps, although 9.6 kbps modems are becoming available. These modems require elaborate circuitry to condense an essentially wide band data spectrum into the 300 Hz to 3300 Hz voice band. Also, when the customer's copper pair is being used for data communication, it is not available for voice communication. Not only does the narrow voice band restrict the rate at which data communication can occur, the use of voice band modems does not allow interactive voice/data transactions to occur. Finally, since voice band data modems use the public switched network; which was designed for voice communications, the degree of performance cannot be guaranteed.
Presently, high speed and high performance data applications require that the customer subscribe to a wide band four wire baseband (digital) data service, such as, Dataphone Digital Service (DDS). In DDS, high performance is obtained by leaving the data un-modulated and transmitting and receiving it through cable equalizers/pre-equalizers. In pending U.S. Pat. application Ser. No. 891,462 filed July 29, 1986, Gupta discloses that by use of a pre- and post-equalizer, the copper cable media can be effectively made wide band. Baseband data in the form of Pulse Amplitude Modulated Nyquist Pulses transmitted through this cable equalizer combination results in good (.gtoreq.70% open) eye performance. Thus, error rate performance better than 10.sup.-8 can be achieved. A thorough treatment of "eye opening" can be found in the book entitled "Data Transmission" by W.R. Bennett and J.R. Davey, page 119, McGraw Hill, 1965.
The problems with this baseband Pulse Amplitude Modulation (PAM) technique are that (1) there is a D.C. component present and (2) there are discrete frequencies present at the baud rate and its multiples. Since telephone copper cable is subject to lightning hits, power crosses, etc., it is highly desirable to interface it through transformers and protection circuitry. The presence of D.C. in the PAM data signal does not allow it to pass through a transformer. Also, energy concentrated at discrete frequency points, can cause cross-talk problems in the other wire pairs in the binder group of which the PAM signal carrying pairs are members. These two problems are solved by use of Alternate Mark Inversion (AMI) pre-coding, wherein the polarity of every alternate "1" is reversed to a "-1". Proof that this eliminates D.C. and discrete frequencies is complex and can be found in "Signal Theory" by L.E. Franks, pages 217-218, Prentice Hall, 1969. Franks demonstrates that the spectra of the AMI/PAM digital signal is of the form (sin Kf)..times.(f) which becomes zero, i.e., D.C. at f=0. In general, the slope at f=0 is not zero and, hence, significant energy is present in the voice band (300 Hz to 3300 Hz). Thus, AMI/PAM and baseband voice transmission are mutually incompatible.
With the explosion of data communication, a need exists for a method and apparatus for transmission and reception of voice and data signals between terminals or nodes over a single pair of standard telephone cable. Present techniques for satisfying this need involve frequency shift keying (FSK) in which two frequencies are required. The error performance of FSK transmission is unsatisfactory for a variety of reasons. The high frequency band is highly attenuated by even moderate length cable and so poor Signal to Noise Ratios (SNR) result leading to degraded performance. Furthermore, since the energy is clustered in narrow bands, cross-talk into other cables in the binder group is created. The more power used for transmission, the greater the cross-talk, limiting the use of other cables in the binder group for wideband services. Therefore, the signal cannot be transmitted as far as one would desire and administrative restrictions have to be placed on mixing other services in the same binder group. Expensive modulation and demodulation circuits are also required for FSK and are a further detriment.